This invention relates to nonwoven fabrics and to fabric laminates which comprise multiconstituent fibers formed from a select combination of polyolefin polymers. The invention more particularly relates to nonwoven fabrics and laminates of the type described having improved fabric properties and processing characteristics.
Nonwoven fabrics produced from spun polymer materials are used in a variety of different applications. Among other uses, such nonwoven fabrics are employed as the cover sheet for disposable diapers or sanitary products. There is considerable interest in making disposable diapers more comfortable and better fitting to the baby. An important part of the diaper comfort is the softness or hardness of the nonwovens used to make the diaper, including the diaper topsheet, barrier leg cuffs, and in some advanced designs, the fabric laminated to the backsheet film. In some diaper designs, a high degree of fabric elongation is needed to cooperate with elastic components for achieving a soft comfortable fit.
One approach to improved diaper topsheet softness is to use linear low density polyethylene (LLDPE) as the resin instead of polypropylene for producing spunbonded diaper nonwoven fabrics. For example, Fowells U.S. Pat. No. 4,644,045 describes spunbonded nonwoven fabrics having excellent softness properties produced from linear low density polyethylene. However, the above-described softness of LLDPE spunbonded fabric has never been widely utilized because of the difficulty in achieving acceptable abrasion resistance in such products. The bonding of LLDPE filaments into a spunbonded web with acceptable abrasion resistance has proven to be very difficult. Acceptable fiber tie down is observed at a temperature just below the point that the filaments begin to melt and stick to the calender. This very narrow bonding window has made the production of LLDPE spunbond fabrics with acceptable abrasion resistance very difficult. Thus, the softness advantage offered by LLDPE spunbonded fabrics has not been successfully captured in the marketplace.
The present invention is based upon the discovery that blending a relatively small proportion of polypropylene of a select class with the polyethylene imparts greatly increased abrasion resistance to a nonwoven fabric formed from the polymer blend, without significant adverse effect on the fabric softness properties. It is believed that the polyethylene and the polypropylene form distinct phases in the filaments. The lower-melting polyethylene is present as a dominant continuous phase and the higher-melting polypropylene is dispersed in the dominant polyethylene phase.
A number of prior publications describe fibers formed of blends of linear low density polyethylene and polypropylene. For example, U.S. Pat. No. 4,839,228 and EP 394,954 teach that useful fibers are formed from blends which are predominantly polypropylene. WO 90/10672 describes that useful fibers are prepared from blends of polypropylene and polyethylene, especially LLDPE, where the ratio of polypropylene to polyethylene is from 0.6 to 1.5. U.S. Pat. No. 4,874,666 describes fibers formed from a blend of LLDPE and high molecular weight crystalline polypropylene of melt flow rate below 20 g/10 minutes. U.S. Pat. No. 4,632,861 and 4,634,739 describe fibers formed from a blend of a branched low density polyethylene blended with from 5 to 35 percent polypropylene.
In accordance with the present invention, nonwoven fabrics and nonwoven fabric laminates are formed from fibers of a select blend of specific grades of polyethylene and polypropylene which give improved fabric performance not heretofore recognized or described, such as high abrasion resistance, good tensile properties, excellent softness and the like. Furthermore, these blends have excellent melt spinning and processing properties which permit efficiently producing nonwoven fabrics at high productivity levels.
The nonwoven fabrics of the present invention are comprised of fibrous material in the form of continuous filaments or staple fibers of a size less than 15 dtex/filament formed of a dispersed blend of at least two different polyolefin polymers. The polymers are present as a lower-melting dominant continuous phase and at least one higher-melting noncontinuous phase dispersed therein. The lower-melting continuous phase forms at least 70 percent by weight of the fiber. The physical and rheological behavior of these blends is part of a phenomenon observed by applicants wherein a small amount of a higher modulus polymer reinforces a softer, lower-modulus polymer and gives the blend better spinning, bonding and strength characteristics than the individual constituents. The lower melting, relatively low molulus polyethylene provides desirable properties such as softness, elongation and drape; while the higher-melting, higher modulus polypropylene phase imparts one or more of the following properties to the dominant phase: improved ability to bond the web; improved filament tie-down (reduces fuzz); improved web propertiesxe2x80x94tensiles, and/or elongation and/or toughness; rheological characteristics which improve spinning performance and/or web formation (filament distribution).
According to one advantageous and important aspect of the present invention, the lower-melting continuous phase comprises a linear low density polyethylene polymer of a melt index of greater than 10 (ASTM D1238-89, 190xc2x0 C.) and a density of less than 0.945 g/cc (ASTMD-792). At least one higher-melting noncontinuous phase comprises a polypropylene polymer with melt flow rate of greater than 20 g/10 min (ASTM D1238-89, 230xc2x0 C).
In one of the preferred embodiments of the invention, the lower-melting continuous phase forms at least 80 percent by weight of the fiber and comprises a linear low density polyethylene having a density of 0.90 -0.945 g/cc and a melt index of greater than 25 g/10 minutes.
In another preferred embodiment, said lower-melting polymer phase comprises linear low density polyethylene as described above and said higher-melting polymer phase comprises an isotactic polypropylene with a melt flow rate greater than 30 g/10 minutes.
In still another preferred embodiment said lower-melting polymer phase comprises at least 80 percent by weight low pressure, solution process, linear short chain branched polyethylene with a melt index of greater than 30 and a density of 0.945 g/cc and said higher-melting polymer phase comprises 1 to 20 percent by weight of isotactic polypropylene.
In another embodiment of the invention, said lower-melting polymer phase comprises linear low density polyethylene with a melt index of 27 and said higher-melting polymer phase comprises an isotactic polypropylene with a melt flow rate of 35 g/10 minutes.
According to another aspect of the present invention, the lower-melting dominant continuous phase is blended with a higher-melting noncontinuous phase of propylene co- and/or ter- polymers. When propylene co-and/or ter- polymers are used as the higher-melting noncontinuous phase, the lower melting continuous phase may be comprised of one or more polyethylenes selected from the group consisting of low density polyethylene, high pressure long chain branched polyethylene, linear low density polyethylene, high density polyethylene and copolymers thereof.